Aryloxyphenoxyalkanecarboxylic acid herbicides (which are also to be understood as meaning heteroaryloxyphenoxy-alkanecarboxylic acid derivatives) are effective grass herbicides. A representative of this class of active substances which is to be mentioned hereinafter is fenoxaprop-ethyl ("FOPE"), which is to be understood as meaning the biologically active D-isomer as well as the racemate. They act on plants from the family of the Poaceae (Gramineae), since only this plant family has a specific form of acetyl coenzyme A carboxylase (ACC) which can be inhibited by micromolar concentrations of FOPE. Remaining terrestrial plants have ACC types whose sensitivity to this class of active substances is 100 to 1000 times lower.
Since FOPE and other aryloxyphenoxyalkanecarboxylic acid herbicides are taken up via the aerial parts of the plants, but are rapidly inactivated in the soil, these herbicides are suitable for controlling grasses post-emergence.
The crop plant maize (Zea mays) is particularly sensitive to FOPE. This is why these compounds cannot be used for controlling grass weeds in maize fields.
It has been found that in areas where FOPE has been applied repeatedly, forms arise spontaneously in populations of wild grasses which are resistant to this class of herbicide. Since such mutations occur only at a rate of approximately 10.sup.-7 to 10.sup.-9, a corresponding search for mutants in maize fields would be of little promise even if maize were not so highly sensitive to FOPE.